At an output stage of an audio apparatus, a power amplifier IC (power amplifier) is generally provided, which makes driving current that is a power-amplified audio signal flow in a speaker. Then, at the output sage of the audio apparatus, an overcurrent detection device or a DC offset detection device is provided in some cases to protect the power amplifier IC (power amplifier) and the speaker from abnormal current due to overcurrent or DC offset (hereinafter, collectively referred to as “abnormal current”). Such abnormal current is relatively large current when caused by short circuit or earth fault, so that it can be easily detected by the overcurrent detection device.
FIG. 18 is a diagram illustrating a relationship between a threshold current for determining overcurrent and driving current by an audio signal in a typical overcurrent detection device. In FIG. 18, current waveform (I1) indicates driving current in normal time, and current waveform (I2) indicates driving current in a case where abnormal current is generated. Driving current (I1) is a current dependent on whether the audio signal is large or small, so a large driving current flows in a large volume period, and a small driving current flows in a small volume period. Then, in the typical overcurrent detection device, threshold current Ith for determining generation of an abnormal current is set using the driving current that flows in the large volume period as a reference. In this case, in the large volume period, a feeble abnormal current is buried in the driving current that is a large current, and an error readily occurs in the flowing driving current due to variation in manufacture of an impedance, a current amplification factor, or the like of a speaker or a power amplifier. Consequently, in a typical overcurrent detection device, detection of the feeble abnormal current is difficult, and threshold current Ith for determining generation of the abnormal current is set so as to be considerably large with respect to the driving current flowing in the large volume period.
In an audio apparatus, there is a case where an impedance of a speaker or a power amplifier, a property of a transistor, or the like is changed due to accumulation of dust or time degradation, and in this case, different from the short circuit current or the earth fault current, feeble abnormal current (I2) continues to flow. Such feeble abnormal current cannot be detected by the above-mentioned overcurrent detection device, so the continuous flow results in heat generation due to resistive loss, causing a problem that a speaker or a power amplifier is damaged.
Against such a background, for example, Patent Literature 1 describes protecting a power amplifier by detecting a DC offset in a constant mute period after a power source is turned on.